ormar/docs/relations/foreign-key.md

ForeignKey

ForeignKey(to, related_name=None) has required parameters to that takes target Model class.

Sqlalchemy column and Type are automatically taken from target Model.

• Sqlalchemy column: class of a target Model primary key column
• Type (used for pydantic): type of a target Model

Defining Models

To define a relation add ForeignKey field that points to related Model.

--8<-- "../docs_src/fields/docs003.py"

Reverse Relation

ForeignKey fields are automatically registering reverse side of the relation.

By default it's child (source) Model name + s, like courses in snippet below:

--8<-- "../docs_src/fields/docs001.py"

Reverse relation exposes API to manage related objects also from parent side.

add

Adding child model from parent side causes adding related model to currently loaded parent relation, as well as sets child's model foreign key value and updates the model.

department = await Department(name="Science").save()
course = Course(name="Math", completed=False) # note - not saved

await department.courses.add(course)
assert course.pk is not None # child model was saved
# relation on child model is set and FK column saved in db
assert courses.department == department
# relation on parent model is also set
assert department.courses[0] == course 

!!!warning If you want to add child model on related model the primary key value for parent model has to exist in database.

Otherwise ormar will raise RelationshipInstanceError as it cannot set child's ForeignKey column value 
if parent model has no primary key value.

That means that in example above the department has to be saved before you can call `department.courses.add()`.

remove

Removal of the related model one by one.

In reverse relation calling remove() does not remove the child model, but instead nulls it ForeignKey value.

# continuing from above
await department.courses.remove(course)
assert len(department.courses) == 0
# course still exists and was saved in remove
assert course.pk is not None
assert course.department is None

# to remove child from db
await course.delete()

But if you want to clear the relation and delete the child at the same time you can issue:

# this will not only clear the relation 
# but also delete related course from db
await department.courses.remove(course, keep_reversed=False)

clear

Removal of all related models in one call.

Like remove by default clear() nulls the ForeigKey column on child model (all, not matter if they are loaded or not).

# nulls department column on all courses related to this department
await department.courses.clear()

If you want to remove the children altogether from the database, set keep_reversed=False

# deletes from db all courses related to this department 
await department.courses.clear(keep_reversed=False)

QuerysetProxy

Reverse relation exposes QuerysetProxy API that allows you to query related model like you would issue a normal Query.

To read which methods of QuerySet are available read below querysetproxy

related_name

But you can overwrite this name by providing related_name parameter like below:

--8<-- "../docs_src/fields/docs002.py"

!!!tip The reverse relation on access returns list of wekref.proxy to avoid circular references.

Relation Setup

You have several ways to set-up a relationship connection.

Model instance

The most obvious one is to pass a related Model instance to the constructor.

--8<-- "../docs_src/relations/docs001.py"

Primary key value

You can setup the relation also with just the pk column value of the related model.

--8<-- "../docs_src/relations/docs001.py"

Dictionary

Next option is with a dictionary of key-values of the related model.

You can build the dictionary yourself or get it from existing model with dict() method.

--8<-- "../docs_src/relations/docs001.py"

None

Finally you can explicitly set it to None (default behavior if no value passed).

--8<-- "../docs_src/relations/docs001.py"

!!!warning In all not None cases the primary key value for related model has to exist in database.

Otherwise an IntegrityError will be raised by your database driver library.